Feature: Option to override the From email address in SMTP relay configuration (#414)

cmd/root.go

@@ -283,6 +283,7 @@ func initConfigFromEnv() {
 	config.SMTPRelayConfig.Password = os.Getenv("MP_SMTP_RELAY_PASSWORD")
 	config.SMTPRelayConfig.Secret = os.Getenv("MP_SMTP_RELAY_SECRET")
 	config.SMTPRelayConfig.ReturnPath = os.Getenv("MP_SMTP_RELAY_RETURN_PATH")
+	config.SMTPRelayConfig.OverrideFrom = os.Getenv("MP_SMTP_RELAY_OVERRIDE_FROM")
 	config.SMTPRelayConfig.AllowedRecipients = os.Getenv("MP_SMTP_RELAY_ALLOWED_RECIPIENTS")
 	config.SMTPRelayConfig.BlockedRecipients = os.Getenv("MP_SMTP_RELAY_BLOCKED_RECIPIENTS")
 

config/config.go

@@ -5,6 +5,7 @@ import (
 	"errors"
 	"fmt"
 	"net"
+	"net/mail"
 	"net/url"
 	"os"
 	"path"
@@ -204,6 +205,7 @@ type SMTPRelayConfigStruct struct {
 	Password                string         `yaml:"password"`           // plain
 	Secret                  string         `yaml:"secret"`             // cram-md5
 	ReturnPath              string         `yaml:"return-path"`        // allow overriding the bounce address
+	OverrideFrom            string         `yaml:"override-from"`      // allow overriding of the from address
 	AllowedRecipients       string         `yaml:"allowed-recipients"` // regex, if set needs to match for mails to be relayed
 	AllowedRecipientsRegexp *regexp.Regexp // compiled regexp using AllowedRecipients
 	BlockedRecipients       string         `yaml:"blocked-recipients"` // regex, if set prevents relating to these addresses
@@ -611,6 +613,15 @@ func validateRelayConfig() error {
 		logger.Log().Infof("[smtp] relay recipient blocklist is active with the following regexp: %s", SMTPRelayConfig.BlockedRecipients)
 	}
 
+	if SMTPRelayConfig.OverrideFrom != "" {
+		m, err := mail.ParseAddress(SMTPRelayConfig.OverrideFrom)
+		if err != nil {
+			return fmt.Errorf("[smtp] relay override-from is not a valid email address: %s", SMTPRelayConfig.OverrideFrom)
+		}
+
+		SMTPRelayConfig.OverrideFrom = m.Address
+	}
+
 	return nil
 }
 

internal/smtpd/relay.go

@@ -9,6 +9,7 @@ import (
 
 	"github.com/axllent/mailpit/config"
 	"github.com/axllent/mailpit/internal/logger"
+	"github.com/axllent/mailpit/internal/tools"
 )
 
 func autoRelayMessage(from string, to []string, data *[]byte) {
@@ -86,6 +87,15 @@ func Relay(from string, to []string, msg []byte) error {
 		}
 	}
 
+	if config.SMTPRelayConfig.OverrideFrom != "" {
+		msg, err = tools.OverrideFromHeader(msg, config.SMTPRelayConfig.OverrideFrom)
+		if err != nil {
+			return fmt.Errorf("error overriding From header: %s", err.Error())
+		}
+
+		from = config.SMTPRelayConfig.OverrideFrom
+	}
+
 	if err = c.Mail(from); err != nil {
 		return fmt.Errorf("error response to MAIL command: %s", err.Error())
 	}

internal/tools/headers.go

@@ -6,6 +6,7 @@ import (
 	"bytes"
 	"net/mail"
 	"regexp"
+	"strings"
 
 	"github.com/axllent/mailpit/internal/logger"
 )
@@ -97,3 +98,63 @@ func UpdateMessageHeader(msg []byte, header, value string) ([]byte, error) {
 
 	return msg, nil
 }
+
+// OverrideFromHeader scans a message for the From header and replaces it with a different email address.
+func OverrideFromHeader(msg []byte, address string) ([]byte, error) {
+	reader := bytes.NewReader(msg)
+	m, err := mail.ReadMessage(reader)
+	if err != nil {
+		return nil, err
+	}
+
+	if m.Header.Get("From") != "" {
+		reBlank := regexp.MustCompile(`^\s+`)
+		reHdr := regexp.MustCompile(`(?i)^` + regexp.QuoteMeta("From:"))
+
+		scanner := bufio.NewScanner(bytes.NewReader(msg))
+		found := false
+		hdr := []byte("")
+		for scanner.Scan() {
+			line := scanner.Bytes()
+			if !found && reHdr.Match(line) {
+				// add the first line starting with <header>:
+				hdr = append(hdr, line...)
+				hdr = append(hdr, []byte("\r\n")...)
+				found = true
+			} else if found && reBlank.Match(line) {
+				// add any following lines starting with a whitespace (tab or space)
+				hdr = append(hdr, line...)
+				hdr = append(hdr, []byte("\r\n")...)
+			} else if found {
+				// stop scanning, we have the full <header>
+				break
+			}
+		}
+
+		if len(hdr) > 0 {
+			originalFrom := strings.TrimRight(string(hdr[5:]), "\r\n")
+
+			from, err := mail.ParseAddress(originalFrom)
+			if err != nil {
+				// error parsing the from address, so just replace the whole line
+				msg = bytes.Replace(msg, hdr, []byte("From: "+address+"\r\n"), 1)
+			} else {
+				originalFrom = from.Address
+				// replace the from email, but keep the original name
+				from.Address = address
+				msg = bytes.Replace(msg, hdr, []byte("From: "+from.String()+"\r\n"), 1)
+			}
+
+			// insert the original From header as X-Original-From
+			msg = append([]byte("X-Original-From: "+originalFrom+"\r\n"), msg...)
+
+			logger.Log().Debugf("[release] Replaced From email address with %s", address)
+		}
+	} else {
+		// no From header, so add one
+		msg = append([]byte("From: "+address+"\r\n"), msg...)
+		logger.Log().Debugf("[release] Added From email: %s", address)
+	}
+
+	return msg, nil
+}

server/apiv1/application.go

@@ -60,6 +60,8 @@ type webUIConfiguration struct {
 		AllowedRecipients string
 		// Block relaying to these recipients (regex)
 		BlockedRecipients string
+		// Overrides the "From" address for all relayed messages
+		OverrideFrom string
 		// DEPRECATED 2024/03/12
 		// swagger:ignore
 		RecipientAllowlist string
@@ -111,6 +113,7 @@ func WebUIConfig(w http.ResponseWriter, _ *http.Request) {
 		conf.MessageRelay.ReturnPath = config.SMTPRelayConfig.ReturnPath
 		conf.MessageRelay.AllowedRecipients = config.SMTPRelayConfig.AllowedRecipients
 		conf.MessageRelay.BlockedRecipients = config.SMTPRelayConfig.BlockedRecipients
+		conf.MessageRelay.OverrideFrom = config.SMTPRelayConfig.OverrideFrom
 		// DEPRECATED 2024/03/12
 		conf.MessageRelay.RecipientAllowlist = config.SMTPRelayConfig.AllowedRecipients
 	}

server/ui-src/components/message/Release.vue

@@ -86,7 +86,13 @@ export default {
 					<div class="row">
 						<label class="col-sm-2 col-form-label text-body-secondary">From</label>
 						<div class="col-sm-10">
-							<input type="text" aria-label="From address" readonly class="form-control-plaintext"
+							<div v-if="mailbox.uiConfig.MessageRelay.OverrideFrom != ''" class="form-control-plaintext">
+								{{ mailbox.uiConfig.MessageRelay.OverrideFrom }}
+								<span class="text-muted small ms-2">
+									* address overridden by the relay configuration.
+								</span>
+							</div>
+							<input v-else type="text" aria-label="From address" readonly class="form-control-plaintext"
 								:value="message.From ? message.From.Address : ''">
 						</div>
 					</div>
@@ -129,11 +135,13 @@ export default {
 					<h6>Notes</h6>
 					<ul>
 						<li v-if="mailbox.uiConfig.MessageRelay.AllowedRecipients != ''" class="form-text">
-							A recipient <b>allowlist</b> has been configured. Any mail address not matching the following will be rejected:
+							A recipient <b>allowlist</b> has been configured. Any mail address not matching the
+							following will be rejected:
 							<code>{{ mailbox.uiConfig.MessageRelay.AllowedRecipients }}</code>
 						</li>
 						<li v-if="mailbox.uiConfig.MessageRelay.BlockedRecipients != ''" class="form-text">
-							A recipient <b>blocklist</b> has been configured. Any mail address matching the following will be rejected:
+							A recipient <b>blocklist</b> has been configured. Any mail address matching the following
+							will be rejected:
 							<code>{{ mailbox.uiConfig.MessageRelay.BlockedRecipients }}</code>
 						</li>
 						<li class="form-text">

server/ui/api/v1/swagger.json

@@ -1948,6 +1948,10 @@
               "description": "Whether message relaying (release) is enabled",
               "type": "boolean"
             },
+            "OverrideFrom": {
+              "description": "Overrides the \"From\" address for all relayed messages",
+              "type": "string"
+            },
             "ReturnPath": {
               "description": "Enforced Return-Path (if set) for relay bounces",
               "type": "string"